Did he control himself? Did HE decide to destroy the planet? Or was Jenova(using the Jenova cells inside him) make him do all that he did?

His actions were not independent. Jenova was just to powerful for him to possibly fight against. He breaks away from her grasp at one point and at one point alone. When he kills Aeris and Cloud starts crying. He llooks down at Cloud and even though you can't tell as good as you culd've if he could actually talk, you can still conclude that from what he said, that he was somewhat sympathetic. Its only that one line, and then Jenova takes over his mind again.

But what do you think?